The present invention relates to ultrasonic imaging systems, and in particular to diagnostic medical ultrasonic imaging systems having adaptive gain in the receive signal path.
It is well known in medical ultrasound imaging that as ultrasonic waves penetrate into a patient's body and return after reflection they are continually attenuated. To compensate for this attenuation, ultrasound imaging systems have traditionally applied a depth dependent gain to the returning echoes. As ultrasonic signals return from increasing depth the applied gain is increased accordingly. This type of gain compensation is generally called depth gain compensation (DGC), or time gain compensation (TGC), because the depth of an echo corresponds to the time taken for the echo to reach a reflecting object and return to the transducer. In this specification, the term depth gain compensation (DGC) will be used to represent this pre-determined range-varying gain function, which preferably does not include the additional gain applied by the user through the commonly called DGC potentiometers (or TGC potentiometers).
Several U.S. patents discuss how to implement DGC, how to predetermine the DGC curve for the selected application, and how to design the system to adjust the DGC curve automatically (see U.S. Pat. No. 4,662,380, U.S. Pat. No. 4,852,576, U.S. Pat. No. 5,313,948, U.S. Pat. No. 5,501,221, U.S. Pat. No. 5,482,045, and U.S. Pat. No. 5,579,768). The common problem with these DGC methods is that as the returning ultrasonic signal is amplified by the DGC, the background noise (the combination of front-end noise and system noise) is also amplified by the same DGC. Since the echo signals from tissue objects decrease in amplitude with depth, but the background noise does not, the output signal-to-noise ratio (SNR) of the image decreases with increasing depth. The penetration limit is often primarily determined by this ratio.
In European patent EP-0843181A1 titled "Variable Compression of Ultrasonic Image Data with Depth and Lateral Scan Dimensions", a means for automatically varying the displayed dynamic range and noise rejection level throughout the image is described.
Thus, a need presently exists for systems that improve the signal-to-noise ratio of medical diagnostic imaging signals at increased depths.